primary and secondary valence
Imagine the central metal in a complex wearing two different kinds of belt. One belt holds ions that can be paid off from a distance, like a debt; the other holds groups gripped tightly right against the body. Werner's great insight was that a metal uses both at once, and they are not the same thing. He called them primary valence and secondary valence.
Primary valence is the ordinary combining power that must be neutralized by negative ions — today we recognize it as the oxidation state of the metal. In [Co(NH3)6]Cl3 the cobalt has a primary valence of three, balanced by three chloride ions sitting outside, free to dissociate in water. Secondary valence is the fixed number of groups the metal holds directly around itself in space — this is what we now call the coordination number, six for cobalt(III). These directly held groups (the six ammonias) are the ligands, and together with the metal they form the coordination sphere, usually written inside square brackets. A clever point: a single anion like chloride can sometimes satisfy both at once — bound directly to the metal (secondary) while also balancing charge (primary).
In modern language we rarely say 'primary and secondary valence'; we say oxidation state and coordination number instead. But the distinction Werner drew is exactly right and still organizes the subject: charge balance (which ions are free counter-ions) is a separate question from geometry (how many groups touch the metal and where). Keeping the two apart is the first skill in reading any coordination formula.
In [CoCl(NH3)5]Cl2 the cobalt has primary valence three (oxidation state +3) and secondary valence six (coordination number 6). One chloride sits inside, bound directly to cobalt — it satisfies both valences at once. The other two chlorides are free outer ions, satisfying only the primary valence, and they precipitate with silver.
A bound chloride can satisfy both the primary valence (charge) and the secondary valence (geometry) at once.
Do not equate primary valence with the net charge of the whole complex ion. Primary valence is the metal's oxidation state; the complex ion's overall charge depends on the metal's oxidation state plus the charges of all bound ligands together.